1. Field of the Invention
The present invention relates to a waveguide path type optical device used in an optical wavelength division multiplex transmission system, and more particular to a waveguide path type polarization independent optical wavelength tunable filter used in a wavelength tunable multiplexer-demultiplexer for multiplexing and demultiplexing any signal optical wavelength.
2. Description of the Related Art
There is proposed, as a conventional filter, an optical wavelength filter wherein a channel type optical waveguide path is provided on a substrate, a surface wave exciting electrode is provided on the substrate through a buffer layer in the vicinity of a middle portion of the input terminal and output terminal of the channel type optical waveguide path, and an element for separating two linearly polarized lights orthogonal to each other is provided in the vicinity of said middle portion and the output terminal (Japanese Patent Application Laid-Open No. 4-51114).
There is also proposed an optical wavelength filter wherein a channel type optical waveguide path is provided on a substrate exhibiting an acousto-optic effect, and a surface wave exciting electrode, a phase shifter electrode and a polarized light separation element are provided on the channel type optical waveguide path (Japanese Patent Application Laid-Open No. 4-159516).
Recently, however, as optical communication systems have been put to practical use, demand rises for a high-capacity, multifunctional, advanced communication system. Also, the addition of new functions of such as generating a higher speed optical signal, multiplexing wavelengths within a single light transmission path, and switching over and changing light transmission paths are required.
In these circumstances, as optical fiber amplifiers have been, in particular, rapidly put to practical use, an optical wavelength division multiplex transmission (WDM) system is being developed actively.
The WDM transmission cannot be realized without a wavelength tunable multiplexer-demultiplexer for multiplexing and demultiplexing any signal optical wavelength. A wide range of wavelength tunable widths, high speed operation and the like are required of an optical wavelength tunable filter used in the wavelength tunable multiplexer-demultiplexer.
As for optical wavelength tunable filters, TE (transverse-electric)-TM (transverse-magnetic) mode conversion type filters using the AO (acousto-optic) effect capable of easily tuning a selected wavelength by changing the frequency of a surface acoustic wave are being developed extensively.
FIG. 1 is a view for showing the structure of a conventional TE-TM mode conversion type waveguide path type polarization independent optical wavelength tunable filter.
The wavelength tunable filter shown therein is obtained by forming a titanium (Ti) diffusion optical waveguide paths 1a, 1b and anisotropic optical waveguide paths 2a, 2b, 2c and 2d are formed on the surface of a substrate 4a of an X-cut lithium niobate (LiNbO.sub.3).
Portions having slightly higher refraction indexes than the substrate 4a become the Ti diffusion optical waveguide paths 1a, 1b and the anisotropic optical waveguide paths 2a, 2b, 2c and 2d. The Ti diffusion optical waveguide paths 1a and 1b are formed by thermally diffusing Ti to the substrate 4a, whereas the anisotropic optical waveguide paths 2a, 2b, 2c and 2d are formed by thermally diffusing Ti to the substrate 4a and then conducting ion (proton) exchange processing.
A surface acoustic wave (SAW) excitation interdigital transducer 5a and surface acoustic wave absorbers 6a and 6b are provided right over the Ti diffusion optical waveguide paths 1a and 1b.
The polarization beam splitter 11a consists of the Ti diffusion optical waveguide paths 1a, 1b and the anisotropic optical waveguide paths 2a, 2b. The polarization beam splitter 11b consists of the Ti diffusion optical waveguide paths 1a, 1b and the anisotropic optical waveguide paths 2c, 2d. The TE-TM mode converter 12a consists of the Ti diffusion optical waveguide paths 1a, 1b and the surface acoustic wave (SAW) exciting interdigital transducer 5a.
Now, the operational principle of this wavelength tunable filter will be described.
First, description will be given to the operations of the TE-TM mode converter 12a of the polarization beam splitter, taking the polarization beam splitter 11a as an example.
FIG. 2 is a view for explaining the operation of the polarization beam splitter 11a.
In FIG. 2, a TE polarization component 15a, which is an extraordinary ray, and a TM polarization component 15b, which is an ordinary ray, of a light 14a incident on the Ti diffusion optical waveguide path 1a are separately introduced to the anisotropic optical waveguide path 2a and the Ti diffusion optical waveguide path 1a, respectively at a polarized light separation basic structural part 10a.
Further, the TE polarization component 15a is multiplexed to the Ti diffusion optical waveguide path 1b at a polarized light separation basic structural part 10b. Both of the polarization components 15a and 15b of the incident light 14a are, therefore, polarization-separation outputted to the Ti diffusion optical waveguide paths 1b and 1a, respectively.
Likewise, as regards a light 14d incident on the Ti diffusion optical waveguide path 1b, a TE polarization component 15g and a TM polarization component 15h of the incident light 14d are separation-polarization outputted to the Ti diffusion optical waveguide paths 1a and 1b, respectively.
It is noted that the TE mode refers to the component of the wave-guided light 14a which electric field is parallel to the substrate, whereas the TM mode refers to the component of the guided wave light 15 which electric field is perpendicular to the substrate.
Next, the operation of the TE-TM mode converter 12a will be described. FIG. 3 is a view for explaining the operation of the TE-TM mode converter.
In FIG. 3, the surface acoustic wave excited by applying an RF signal 13a from an oscillation circuit to the interdigital transducer 5a acts as a periodic refractive index grating for the wave-guided light 15a and 15b.
In this case, it is assumed that the guided wave 15a has only a TE polarization component and the guided wave 15b has only a TM polarization component.
By using the X cut lithium niobate substrate and setting the direction of optical wave transmission to Y direction, the Ti diffusion optical waveguide paths 1a and 1b differ in the effective refractive index of both the TE and TM modes.
If the following phase matching conditional equation (1), where the refractive index grating period formed by the surface acoustic wave is .LAMBDA. and the effective refractive indexes of the TE mode and TM mode are NTE and NTM, respectively, is satisfied, a wavelength .lambda. is subjected to TE-TM mode conversion by interaction with the refractive index grating. EQU .lambda.=.vertline.NTE-NTM.vertline..LAMBDA. (1)
The refractive index grating period .LAMBDA. is inversely proportional to the frequency f of the RF signal 13a. Due to this, it is possible to change easily wavelength .lambda. to be subjected to TE-TM mode conversion by changing the frequency of the RF signal 13a. Thus, by appropriately setting the frequency of the RF signal 13a, any wavelength can be subjected to TE-TM mode conversion.
In case of the mode converter 12a, therefore, the TE polarization wave-guided light 15a, only if it is the wave-guided light of a wavelength satisfying the phase matching condition with respect to the refractive index grating period .LAMBDA., is TE-TM mode converted to a TM polarization wave-guided light 15c on the Ti diffusion optical waveguide path 1b.
On the other hand, on the Ti diffusion optical waveguide path 2a, the TM polarization guided wave 15b, only if it is a guided wave of a wavelength satisfying the phase matching condition with respect to the refractive index grating period .LAMBDA., is TE-TM mode converted to a TE polarization wave-guided light 15d.
Based on the operations of the above-stated polarization beam splitters 11a, 11b and the mode converter 12a, the operational principle of the wavelength tunable filer will be described with reference to FIG. 1.
In FIG. 1, the light 14a applied from an input port 3a is separated into the TE polarization component 15a and the TM polarization component 15b introduced to the Ti diffusion optical waveguide paths 1b and 1a, respectively, by the polarization beam splitter 11a.
Only the wavelength which satisfies the phase matching condition by interaction with the refractive index grating .LAMBDA., of the TE polarization component 15a incident on the Ti diffusion optical waveguide path 1b is converted from the TE polarization component 15a to a TM polarization component 15c by the TE-TM mode converter 12a. Thereafter, the resultant TM polarization component 15c is introduced to the Ti diffusion optical waveguide path 1b by the polarization beam splitter 11b and is outputted as the TM polarization component of the light 14c from a port 3d.
The TE polarization component 15a which has not been subjected to mode conversion is introduced to the Ti diffusion optical waveguide path 1a by the polarization beam splitter 11b and outputted as the TE polarization component of the light 14b from a port 2c.
Meanwhile, only the wavelength which satisfies the phase matching condition by interaction with the refractive index grating .LAMBDA., of the TM polarization component 15b incident on the Ti diffusion optical waveguide path 1a is converted from the TM polarization component 15b to a TE polarization component 15d by the TE-TM mode converter 12a as in the same manner as stated above. Thereafter, the resultant TE polarization component 15d is introduced to the Ti diffusion optical waveguide path 1b by the polarization beam splitter 11b and outputted as the TE polarization component of the light 14c from the port 3d.
The TM polarization component 15b which has not been subjected to mode conversion is introduced to the Ti diffusion optical waveguide path 2a by the polarization beam splitter 11b and outputted as the light 14b from the port 3c. By appropriately setting the frequency of the RF signal 13a, therefore, it is possible to select only a desired wavelength as an outgoing light 14c from the port 3d no matter how the input light 14a is polarized. It is also possible to output only non-selected light as the outgoing light 14b from the port 3c.
FIG. 4A is an explanatory view showing an example of light intensity attenuated wavelength characteristics (to be referred to as "filter characteristics" hereinafter) of the output light 14c from the port 3d of the conventional waveguide path type polarization independent optical wavelength tunable filter stated above. FIG. 4B is an explanatory view showing an example of filter characteristics of the output light 14b from the port 3c thereof.
The problem with the conventional filter is, however, that the attenuation of the non-selected light intensity in respect of the light intensity of the selected central wavelength does not exceed a certain level throughout wavelength bands in the filter characteristics of the selected light 14c. The reason for the problem will be described with reference to FIG. 5 as shown below.
In FIG. 5, the TM polarization component 15c of the selected light, which has been subjected to mode conversion as stated above, is introduced to the Ti diffusion waveguide path 1b. At the same time, at the polarized light separation basic structural part 10a, the TE polarization component 15e of the non-selected light with polarized light separation quantity according to the control accuracy of, for example, an actual production process is incident on the Ti diffusion optical waveguide path 1b.
Likewise, on the Ti diffusion optical waveguide path 1a, the TE polarization component 15d of the selected light which has been subjected to mode conversion passes through the anisotropic optical waveguide path 2b and introduced to the Ti diffusion optical waveguide path 1b. At the same time, at the polarized light separation basic structural part 10d, the TM polarization component 15f of the non-selected light with polarized light separation quantity according to the control accuracy of, for example, an actual production process, is incident on the Ti diffusion optical waveguide path 1b.
As a result, the TE polarization component 15e of the non-selected light incident on the Ti diffusion optical waveguide path 1b and the TM polarization component 15f of the non-selected light become noise components for the selected light, thereby deteriorating attenuation characteristics. The noise components are resulted from the polarized light separation characteristics of the polarization beam splitter according to the control accuracy of, for example, an actual production process. For that reason, it is difficult to improve filter characteristics.